1. Field of the Invention
The present invention relates to an image forming apparatus that has a large size display screen and to a method for manufacturing such an image forming apparatus. More specifically, the present invention relates to an image forming apparatus that is designed by arranging a circuit board where electric wiring is provided in a so-called vacuum container, in which pressure is substantially reduced, and a method for manufacturing such an image forming apparatus.
2. Related Background Art
Recently, a light, thin image forming apparatus, a so-called flat display, is attracting attention as the replacement for a large, heavy Braun tube. As such a flat display unit, a liquid crystal display has been enthusiastically studied and developed; there are, however, problems remaining for the liquid crystal display that an image is dark and an angle for field of view is narrow. As a replacement for the liquid crystal display there is a self-emitting flat display, i.e., a plasma display panel (PDP), a fluorescent display tube (VFD), or a multi-electronic source flat type display panel.
When compared with a liquid crystal display, a self-emitting flat display provides a brighter image and a larger field of view angle. However, since such a flat display is so designed that a substrate where functional components and electric wiring are provided is arranged in a so-called vacuum container, in which air pressure is substantially reduced, a technique is required that can provide a stable performance for the flat display for an extended time of a period. When the wiring for an electronic circuit is to be produced, generally a thin film is formed on a workpiece, such as a substrate, and patterning is performed on the resultant structure. For example, with such one method that is employed, after an A1 material has been deposited on the substrate, a wiring pattern is formed for photolithography and etching. Since the procedures of photolithopraphy and etching are complex, a method for forming a wiring pattern without using those procedures is disclosed in Japanese Patent Application Laid-Open No. 3-142894. With the disclosed method, printing is performed directly on a substrate by using an organic metal ink to describe a pattern, followed by electrolytic metal plating of the pattern to provide a metal film of 0.5 to 3 .mu.m. According to the method disclosed in this application, close adhesion of a fine pattern is increased and a sheet resistance of the fine pattern is reduced. While in the publication an explanation is given for the application of the method for a printer head, an image sensor, and a hybrid IC, there is no description for its application for a self-emitting flat display that is so designed that a substrate, whereon functional components and electric wiring are provided, is arranged in the above described vacuum container.
As a self-emitting flat display, a flat image forming apparatus that employs a multi-electronic source to cause a phosphor to become luminescent will now be described.
Conventionally a surface conductive emitter, which is described in a report by M. I. Elinson, Radio Eng. Electron Phys., 10 (1965), is known as an element with a simple structure that can emit electrons. This emitter employs a phenomenon whereby the emission of electrons occurs when, in parallel to the film face, a current is supplied to a thin film that is deposited on a substrate and that has a small dimension.
Reported as such surface conductive emitters are an element with SnO.sub.2 thin film deposited, as in the report by Elinson, an element with Au thin film deposited (G. Dittmer, Thin Solid Films, 9, 317 (1972)), an element with In.sub.2 O.sub.3 /SnO.sub.2 thin film deposited (M. Hartwell and C. G. Fonstad, IEEE Trans. ED Conf., 519 (1975)), and an element with carbon thin film deposited (Araki et al., Vacuum, Vol. 26, No. 1, p. 22 (1983)).
The arrangement of the above Hartwell element is illustrated in FIG. 15 as one specific arrangement of such a surface conductive emitter. In FIG. 15, reference number 101 denotes an insulating substrate, and 102, a thin film for forming an electron emission portion, which is, for example, a H-shaped metal oxide thin film that is deposited by sputtering. Conductive processing called forming, which will be described later, forms an electron emission portion 103.
Conventionally, according to the general method for making a surface conductive emitter, before the emission of electrons, conductive processing called forming is performed in advance on the thin film 102 to form an electron emission portion, and the electron emission portion 103 is formed. More specifically, the forming is a process during which a voltage is applied to both ends of the thin film 102 to cause local damage, deformation, or deterioration of the thin film 102, and the electron emission portion 103 that has a high resistance to electricity is provided. In the electron emission portion 103, part of the thin film 102 is fractured and electrons are emitted in the vicinity of the fractured area.
Disclosed in U.S. Pat. No.5,066,883 is an innovative surface conductive emitter where between the element electrodes are dispersed and located particles that permit the emission of electrons. This electron emitter can control the positioning of electron emission portions more accurately than the conventional surface conductive emitters, making it possible for electron emitters to be arranged more accurately. A specific arrangement for such a surface conductive emitter is shown in FIG. 16. In FIG. 16, reference number 201 denotes an insulating substrate; 202 and 203, element electrodes for electric connection; and 204, a thin film that is made of an electron emission particle material that is dispersed and positioned.
For the surface conductive emitter, an appropriate electrode interval between the paired electrodes 202 and 203 is 0.01 micron to 100 microns, and an appropriate a sheet resistance for the electron emission portion in the thin film 204 is 1.times.10.sup.3 .OMEGA./.quadrature. to 1.times.10.sup.9 .OMEGA./.quadrature..
When the above described surface conductive emitter is employed as a flat display, it must be located in a vacuum container because an electron beam is irradiated. In the vacuum container, a face plate is positioned above and almost perpendicularly to the emitter to provide an electron emitting device. When a voltage is applied between the electrodes, a phosphor is irradiated by an electron beam, which is acquired from the electron emission portion, in order to cause the phosphor to become luminescent, making it possible for the emitter to be used as a flat display device.
When the screen size of the above described flat display device has been increased, however, the following shortcomings have arisen. Specifically, for manufacturing a thus structured surface conductive emitter, a functional thin film is deposited on a workpiece and patterning is performed on the resultant structure. When the photolithographic technique is employed to produce a fine pattern on a large substrate that is, for example, 40 cm square or larger, a large manufacturing apparatus that includes an aligner is required and the manufacturing costs are enormous.
Further, unlike an aligner that is employed for silicon semiconductors, it is difficult for an aligner that handles large substrates to set a pattern processing size to 4 microns or smaller because of optical limits and because a shorter processing time is required for each substrate. The production of a display device that requires much finer patterns is difficult.
In addition, for a large substrate of about one meter square, it is difficult to increase the size of the manufacturing device itself. Even if a large device that can be used for exposing could be provided, the processing for each substrate would take longer and the manufacturing costs would be greatly increased.
As other methods for processing an electronic circuit, there may be employed a screen printing method, or a method where pattern printing is performed by using a conductive paste or an insulating paste and then annealing the resultant structure to form an electrode wiring pattern and an insulation layer. The patterning that involves the use of a printing method can be employed for comparatively large substrates, and the processing time that is required for each substrate is shorter than that which is required for the photolithographic technique.
However, a printed pattern tends to be deformed due to the flowability of resist ink, of a conductive paste or of an insulating paste, the generation of blank areas and the poor transfer of a print pattern, and the pressure exerted by a print pattern. Therefore, delicate control of a pattern meter and skill are required to maintain the high accuracy in the size of pattern. When wiring is formed by printing, that wiring is comparatively inferior in its density. When the surface is enlarged and examined, it is found to be comparatively porous. When such wiring that has inferior density is to be applied to the above described self-emitting flat display, since the circuit substrate with such wiring is positioned in a vacuum container, there are problems, such as the adsorption of gas or the discharge of gas by wiring that has less density, the change in the degree of vacuum due to the gas discharge, and the deterioration of the display performance.
Further, to increase the size of the display screen of a flat image-forming apparatus, the length of the drive wiring that is arranged in the screen is extended, and in consonance with the length of the wiring, wiring resistance is increased between a wiring electrode end, to which a voltage is applied, and a wiring electrode end that is opposite it.
The following problems may occur, depending on the amount of increase in wiring resistance:
1) A voltage drop relative to the applied voltage occurs, and accordingly, voltages that are applied between the connected elements are different at both ends of wiring are different, so that a difference in the display luminescence is incurred and an uneven image tends to be produced.
2) A time lag occurs between transmitted element drive signals, and the time when a drive signal is provided at the connected elements varies at both ends of wiring. Therefore, for image displaying on a large screen, the period of time for the display of one screen frame is extended and a displayed image is unnatural and is not visually smooth.
Thus, the reduction of wiring resistance must also be considered.
Additionally, wiring formed by printing has a resistivity higher than that of wiring formed by photolithography, so the wiring must be made thick for a large size flat image forming apparatus. For this reason, the wiring must be arranged in consideration of the influence on the traveling process of electrons emitted from the electron emitter.